Flushing devices are used to supply water to the water intake ports of outboard motors and stern drives to clean the internal passages of the motors or to provide coolant to allow the motors to be run while they are out of the water. Exemplary of such devices are the flushing accessory disclosed in U.S. Pat. No. 3,931,828 to Lawler issued on Jan. 13, 1976 and U.S. Pat. No. 4,052,953 to Patel issued on Oct. 11, 1977. The Lawler device uses a U-shaped body straddling the outboard propulsion unit to hold resilient sealing cups over the water inlets, while the Patel device uses a strap girdling the propulsion unit to hold a sealing cup over the water inlet. With these devices, considerable effort must be taken to assure that the sealing cups are accurately positioned over the water inlets to avoid water leakage and prevent air from being drawn into the cooling system.